The present invention relates to a composition suitable for making films, wherein the composition contains keratin obtained from avian feathers and at least one OH containing plasticizer. The present invention also relates to protein-based films containing at least one protein and at least one OH containing plasticizer, wherein the protein is a protein whose PE value is ≧2.5 where PE=(S+T+Y)/C where PE is plasticizer efficiency and where S, T, Y, and C are the amino acids serine, threonine, tyrosine, and cysteine, respectively, in the protein.
There has been much recent interest in developing biopolymer materials for many different applications. Biopolymers, whether natural or synthetic, are biocompatible and therefore appropriate for biomedical applications such as implantation or drug delivery. In addition, biopolymers are being considered as alternatives to commodity synthetic polymers because the biopolymers are biodegradable or environmentally-friendly. Biopolymers from sustainable resources would have a distinct advantage over petroleum-derived polymers in this respect. Naturally derived biopolymers such as gelatin, soybean and wheat proteins, and sunflower proteins have been processed into films using a variety of techniques. These techniques include solvent-cast films as well as thermally-processed films. Solvent casting is tedious and if the solvents are volatiles would defeat the purpose of environmental-friendliness. Thermal processing is simpler and is the method currently used in industry. If biopolymers from sustainable resources are to be used commercially, the biopolymers have to be processed through preferred processing methods. It becomes imperative to identify biopolymers from sustainable resources that can be easily processed using available technology.
Keratin is a biologically important protein because it comprises most of the outer layers of animals. Keratin can be found in hair, nail, epidermis, hoof, horn, and feather (Vincent, J., Structural Biomaterials, Princeton University Press, 1990). Keratin is a unique protein because it contains a large amount of cysteine relative to other proteins (Fraser, R. D. B., et al., Keratins: Their Composition, Structure, and Biosynthesis, Charles C. Thomas Publisher, 1972, p. 31). Cysteine is a sulfur-containing amino acid and can form sulfur—sulfur (S—S) bonds or “cross-link” with other intra- or inter-molecular cysteine molecules. The cross-links plus other protein structural features, like crystallinity and hydrogen-bonding, give keratin very high physical properties (Fraser, R. D. B., and T. P. MacRae, Molecular structure and mechanical properties of keratins, Symposia of the Society for Experimental Biology, Number XXXIV: The Mechanical Properties of Biological Materials, Cambridge University Press, 1980, p. 211–246). The amount of cysteine varies depending on the keratin source. Wool keratin contains about 15% cysteine while feather keratin contains about 7% cysteine (Fraser, R. D. B., et al., 1972; K. M. Arai, K. M., et al., Eur. J. Biochem., 132: 501 (1983)).
Thermally processing natural keratin is difficult because of the permanent cross-links. The keratin must be reduced (i.e., covalent sulfur—sulfur bonds must be broken) to get a soluble fraction for further processing. There are many techniques to reduce keratin (Schrooyen, P. M. M., et al., J. Agric. Food Chem., 48: 4326–4334 (2000)). Acid and alkali hydrolysis, alkaline sodium sulfide treatment, enzymatic treatment, and ammonium copper hydroxide treatment result in S—S reduction and peptide bond breakage. The sulfur—sulfur reduction is an advantage but the peptide bond breakage is a disadvantage. Sulfitolysis with performic acid and use of thiols in concentrated urea solutions at alkaline pH will selectively reduce S—S bonds without peptide bond breakage. The thiol technique appears to be the currently preferred method because the S—S bonds can re-form easily after processing (Schrooyen, P. M. M., et al., J. Agric. Food Chem., 49: 221–230 (2001)). However, reduction requires multiple chemical treatment steps, sufficient time for reaction, and subsequent processing to eliminate the chemicals used for treatment. Therefore, reduction of even small amounts of keratin requires hours to days.
We have found that treating natural keratin (e.g., from feather quill and fiber) with OH containing compounds (e.g., glycerol) surprisingly allows for the treated protein to be pressed into films at typical polymer processing temperatures. Relatively clear, cohesive films are easily formed. No reduction or oxidation agents are used in the process.